The use of air conditioners during the summer months requires large quantities of energy primarily during the daytime hours when other forms of energy consumption are also high. Peaking generators are required to increase electricity generating capacity so that power loads are met. However, energy consumption decreases dramatically at night, and the peaking generators are not needed. Thus, the expensive peaking generators are run only half the time, decreasing the efficiency of the power facility. To alleviate this problem, thermal energy storage systems, which can utilize off peak, night-time electricity have been proposed.
Thermal energy storage systems contain a cooling medium, which is frozen during the off peak, evening hours. During the daytime, heat from the surrounding area is used to melt the cooling medium. The removal of heat to drive the decomposition causes the surrounding area to become cooler.
The use of ice and water as the cooling medium is an ancient practice Even today, the third world countries still use ice and water to condition air in the summer. However, the systems using water are inefficient and bulky. Extraction of sensible heat is inefficient due to the specific heat of water. Ice is also an undesirable heat extraction medium because the low freezing temperature requires large quantities of energy for refrigeration.
Hydrate systems, such as Na.sub.2 SO.sub.4 .multidot.10H.sub.2 O have also been disclosed. However, in operation, the hydrates tend to segregate after a few cycles. In addition such hydrates are inefficient and corrosive to the system.
U.S. Pat. No. 4,540,501 discloses a thermal energy storage system which uses clathrates as the cooling medium. Clathrates are hydrates which use a non-stoichiometric number of water molecules per guest molecule. The guest molecule fills the interior of the lattice, stabilizing the clathrate. This stabilization allows the water lattice structure to form at temperatures significantly higher than the temperature of ice formation (0.degree. C.). The guest molecule must be highly insoluble in water, and must have a molecular size which is less than 7 .ANG.. In U.S. Pat. No. 4,540,501 the guest molecule is a refrigerant chosen from brominated, chlorinated and fluorinated hydrocarbons including CCl.sub.2 F.sub.2, CCl.sub.3 F, CBrF.sub.3, CHCl.sub.2 F, CHClF.sub.2, CH.sub.2 ClF and CH.sub.3 CClF.sub.2. The surfactant used is generally described as a nonionic fluorosurfactant having the chemical formula F(CF.sub.2 CF.sub.2).sub.3-8 CH.sub.2 CH.sub.2 O(CH.sub.2 CH.sub.2 O).sub.9-11 H.
The halogenated hydrocarbons which are used as the guest molecules are not water miscible. Clathrates will not form unless the guest and host (lattice) compounds are in contact. In an attempt to bring the guest molecule and water into closer contact, various surfactants have been added. However, for the known guest molecules relatively large quantities of surfactant are required (on the order of 300 ppm) and some of the guest molecule will associate with the surfactant instead of forming a clathrate with water. This decreases the efficiency of the thermal energy storage system.
Accordingly, it is the goal of the present invention to provide guest molecule-surfactant combinations with good energy storage capacity (high disassociation energy and high decomposition temperatures) which minimize the amount of guest molecule which associate with the surfactant. Particularly, it is the goal of the present invention to find surfactants which at levels much lower than 300 ppm can effectively form an emulsion between the water and the guest molecules. Such a cooling medium would increase the overall efficiency of the thermal energy storage system.
Furthermore, many of the guest molecules presently being used are CFCs such as trichlorofluoromethane (CFC-11). The use of these compounds is becoming disfavored because of the detrimental effect to the ozone layer. Thus it is a goal of the present invention to find a cooling medium which poses less of a threat to the ozone layer. Halohydrocarbons such as HCFC 141b which contain hydrogen, and are believed to pose less of a threat to the ozone layer, and are thus proposed as the guest molecule in clathrate formation according to the present invention.